1. Field of the Invention
The present invention relates generally to communication systems that transmit, receive and process communication signals and, more particularly, to a system and method for providing multi-beam scheduling in a communication system.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Communication systems that transmit and receive communication signals continue to grow in importance. Such systems are used to provide television, radio, satellite communication, cell phone service, wireless computing networks and the like. An important aspect of such systems is the ability to efficiently process signals to continue to improve the quality of service provided to users.
Channelization Codes in Code Division Multiple Access (CDMA) Systems
Cellular telephone systems may employ a variety of communication schemes to transmit data from a base station to a pool of mobile users within the transmission range of the base station at any given time. The area within the transmission range of a base station may be referred to as a “cell.” One of the schemes employed to transmit data efficiently in a cell is known as code division multiple access (CDMA).
In a CDMA cell, one or a few users are chosen (scheduled) at each time instant in which data are transmitted. The choice of users for scheduling may be based on several factors such as the instantaneous channel strength of the users, the amount of data that the users have waiting in their buffers and the like. The scheduled users share the system resources available in that time slot for transmission of their respective signals. The potentially scarce shared system resources that are required for transmission of users' signals, and affect their quality of service metrics within the system include transmission power, channelization codes and the like.
In general, channelization codes are codes that may be used to create signals that may be distinguished from each other so that signals intended for reception by a particular user are not confused with signals intended for reception by other users. In other words, a channelization code may be employed to impart a uniquely identifiable pattern to each signal being transmitted by a base station. Channelization codes may be employed to impart particular transmission characteristics to signals as well. For example, different channelization codes may be employed to impart orthogonality to signals transmitted simultaneously within the same cell to prevent those signals from interfering with each other. Orthogonal signals are signals that do not interfere with the information contained in each other upon propagation through a channel that does not distort the signals. The use of channelization codes to provide orthogonal signals within the same cell is an example of conservation of system resources because the use of orthogonal signals allows greater amounts of data to be transmitted per unit of time.
The use of channelization codes to allow the simultaneous transmission of non-interfering data is called code multiplexing. Generally speaking, a signal that is being transmitted to any user in a cell at the same time must have its own set of channelization codes to prevent interference with other simultaneously transmitted signals. Thus, the number of channelization codes available at any particular time may be a limitation on the amount of data that may be transmitted within a system at any given time.
Beamforming and Fixed Beam Networks
Antennas and beamformers are important components that assist in the effective transmission and reception of transmitted communication signals. An antenna assembly typically transmits communication signals and receives transmitted communication signals from the air. Many antennas have arrays that are comprised of multiple elements to assist in the transmission and reception of communication signals.
Antenna arrays may be used to perform beamforming to enhance reception of signals from different angles of arrival, and transmit beamforming to enhance the quality of transmission of signals to different users. Phase offsets between signals received from a user on different elements of the antenna array depend on the angle of arrival of the user's signals at the antenna array. This phenomenon can be utilized to combine signals arriving from a desired direction constructively at the base station receiver using a receive beamformer. A receive beamformer is a device that receives inputs from the various elements of an antenna array and combines them into output signals or beams based on certain criteria.
In addition, transmit beamformers may be used to enhance signals prior to their transmission by an antenna array. Transmit beamformers may apply weighting coefficients to the signal intended for any user before transmission by an antenna array such that the desired signal strength for the user is enhanced and/or that the interference caused by this user's signal to other users is reduced. The weighting coefficients applied by a transmit beamformer may be adjusted according to various measurements of the signals received from the desired user at the antenna array or any other knowledge of the user's angle of location from the antenna array. Using transmit beamforming weight coefficients, the signal intended for a desired user may be thought of as being “steered” toward the direction of the desired user, such that the signals strength for the desired user is maximized and interference caused by this signal to users located at other angles is reduced.
Beamformers may be employed to create configurations known as fixed beam networks. In a fixed beam network or system, a beamformer may be adapted to provide a plurality of beams by maximizing signal strengths towards a plurality of fixed or predefined directions. A set of beamformer weight coefficients are stored for each fixed beam serving a specific portion of the cell. When users are within the proximity of one of the fixed beams, their signal reception from the base station is strong. The beamforming weight coefficients that are applied for any user in the system are chosen to be the coefficients corresponding to the strongest fixed beam for that user. Fixed beam networks are generally a compromise between complexity and performance, since they are generally easier to implement than dynamically computing beamforming weight coefficients individually for each user in the system.
Sectorization is a strategy for attempting to improve the use of system resources in a fixed beam network. As an example, we may compare a hypothetical three-cell deployment scenario where the coverage area of a base station is split into three cells, each covering a 120 degree angular region, to a six-cell deployment scenario with size cells, each covering a 60 degree region. Each cell in either deployment scenario may employ its own set of channelization codes that may be used to schedule users that are within its coverage. A base station in a six-cell system therefore may use twice the total number of channelization codes of a base station in a three-cell system. Sectorization, however, may be difficult to implement without significantly changing antenna configurations.